1. Field of the Invention
The invention relates to an image forming apparatus comprising a guide rail comprising a guide surface extending in a predetermined direction, a carriage disposed on the guide surface of the guide rail, a recording head mounted on the carriage, and a belt drive mechanism comprising a timing belt and configured to cause the timing belt to move in order to reciprocally move the carriage along the guide surface.
2. Description of Related Art
A known inkjet-type image forming apparatus records an image on a recording medium by selectively ejecting ink droplets from a recording head onto the recording medium. The recording head is mounted on a carriage, which is supported by a guide rail and a guide shaft and moved reciprocally in a direction perpendicular to a direction in which a recording medium is fed (hereinafter referred to as “a medium feeding direction”). While the carriage is moved reciprocally, the ink droplets are ejected selectively from the recording head onto the recording medium.
The carriage is moved reciprocally by a belt drive mechanism having a timing belt. For example, the timing belt may be stretched between two pulleys, and the timing belt may be connected to the carriage. The pulleys may be driven by a drive source, such as a motor, and rotated. Receiving the rotational driving force of the pulleys, the timing belt may be driven continuously about the two pulleys. Thus, the carriage fixed on the timing belt may be moved reciprocally on the guide rails.
FIG. 26 is a schematic view of a carriage 202 to be moved reciprocally on a guide rail 201. Guide rail 201 is a flat plate having a substantially horizontal upper surface, and supports carriage 202 in such a manner that guide rail 201 reciprocates carriage 202. Pulleys 203 and 204 are disposed at either ends of guide rail 201, and a timing belt 205 extends therebetween. Carriage 202 has a holding portion 206 that holds timing belt 205. Thus, carriage 202 is connected to timing belt 205, and moved reciprocally on guide rail 201 by circumferential movement of the timing belt 205 about pulleys 203 and 204.
As shown in FIG. 26, there is a height difference H between the height of timing belt 205 at pulleys 203 and 204 and the height of timing belt 205 at holding portion 206. In particular, holding portion 206 holds timing belt 205 at a position higher only by difference H than pulleys 203 and 204. Thus, at holding portion 206, tension is applied upwardly to timing belt 205, this tension acts vertically, thereby preventing carriage 202 from being urged toward guide rail 201 and lifted from guide rail 201 during reciprocal movement.
As shown in FIG. 27, even when the carriage 202 is moved to pulley 203, difference H is unchanged. Nevertheless, compared with that carriage 202 is located substantially centrally between pulleys 203 and 204, a distance from pulley 203 to holding portion 206 is reduced, and the angle of inclination of timing belt 205 changing from the height at pulley 203 to the height of holding portion 206 becomes increased. Thus, the urging force applied vertically due to the tension of timing belt 205 increases near pulley 203 or 204 when compared with the urging force at the substantially central position between pulleys 203 and 204. Namely, when carriage 202 is located near pulley 203 or 204, friction between timing belt 205 and pulley 203 or 204 increases. As a result, timing belt 205 may become worn on one side.
The torque of motor to rotate pulleys 203 and 204 is set based on a location at which friction is increased, i.e., based on when carriage 202 is located near pulley 203 or 204. When the friction between pulley 203 or 204 and timing belt 205 and the friction between guide rail 201 and carriage 202 increases, a motor with significant torque is required. To reduce the friction when carriage 202 is located near pulley 203 or 204, pulleys 203 and 204 may be disposed at positions away from the ends of travel between which carriage 202 is moved reciprocally, in order to extend the distance from pulley 203 to holding portion 206. Nevertheless, in this case, a width of the apparatus in a direction in which carriage 202 is moved reciprocally may be increased, thereby preventing or inhibiting a reduction in the size of the apparatus.